Presence of labeled monocytes, macrophages and microglia in a stab wound of the brain following an injection of bone marrow cells labeled with 3H‐uridine into rats

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Abstract
To clarify the origin of the cells appearing in a stab wound of the parietal cortex, bone marrow cells from inbred Lewis rats were labeled in vitro with 3H‐uridine and injected intravenously into 36 rats of the same strain Just before or after the brain had been stabbed. The animals were sacrificed from 1 to 15 days after the injection of labeled cells. The cells in the wound were examined and the percentage of the various types recorded. Radioautographs were prepared of the wound and the surrounding neuropil, as well as of the corresponding region on the intact side; and they were searched for labeled cells.One day after the stabbing, the wound consisted of a central region containing fluid and red blood cells, and a marginal region of disorganized nervous tissue including granulocytes, monocytes and macrophages. Around the wound, the neuropil appeared normal, except for the perivascular areas appearing more cellular and distended than normally.During the next few days, the granulocytes and monocytes gradually disappeared from the wound. Meanwhile, the macrophages, which initially often showed monocytic features, became increasingly numerous. However, after the seventh day, the macrophages decreased in number, while many small cells appeared in the wound. These displayed a scanty basophilic cytoplasm rich in phagosomes and a nucleus with dense chromatin clumps similar to that seen in microgliaA search of the radioautographs revealed no labeled cell on the intact side, but 220 labeled cells on the injured side. Most of these cells were within the wound itself, and a few in the surrounding neuropil. Within the wound, labeled granulocytes and monocytes were common one day after injection, but disappeared after the first three days. Few labeled macrophages were observed at one day but many at three days; none were labeled after the seventh day, however, possibly due to turnover and loss of the 3H‐uridine labeled RNA.In the neuropil surrounding the wound, a few labeled cells identified as microglia were observed during the first three days after injection. They were located in perivascular areas, in satellite position to neurons and within the nearby subependymal layerIt is concluded that, under the influence of a stab wound, blood cells of bone marrow origin enter the wound and the surrounding neuropil. The presence of macrophages with monocytic features within the wound indicates that the monocytes transform into macrophages. The small cells displaying phagosomes and a microglia‐like nucleus, which become numerous after the seventh day, are tentatively interpreted as intermediates in the transformation of the monocytederived macrophages into microglia. Finally, the early labeled cells identified as microglia in the surrounding neuropil are interpreted as a direct transformation of monocytes into microglia.